Why you should stop using the term ‘Deadman’

The Deadman Control

Do you use the phrase ‘dead­man’ or ‘dead­man switch’ when talk­ing about safety related con­trols on your machinery? I often run into this when I’m work­ing with cli­ents who use the terms to refer to ‘enabling devices’ – you know, those two or three-​position switches that are found on robot teach­ing pendants and in oth­er applic­a­tions to give the oper­at­or a way to stop machinery, even if they have already been injured or killed by the equip­ment. Calling these devices a ‘Deadman Switch’ or even a ‘Live-​Man Switch’ as the three-​position devices are some­times called, sends entirely the wrong mes­sage to the user as far as I’m con­cerned. The object­ive of our work as machinery safety engin­eers is to pre­vent injur­ies from hap­pen­ing in the first place. Using a device that is designed to determ­ine if the user is dead or uncon­scious means someone screwed up!

A little history

The term ‘dead­man’ comes from a device that was developed in the 1880’s by pion­eer­ing elec­tric­al engin­eer Frank Sprague. Sprague was work­ing on elec­tric trac­tion motor tech­no­logy, using these new machines to power street rail­ways (street­cars) and elec­tric elev­at­ors. The early DC motor con­trols used in both street­cars and elev­at­ors required an oper­at­or. The oper­at­or used a hand con­trol to move the street­car for­ward or back­ward along the track and to con­trol the speed of the car. In elev­at­ors, the oper­at­or used a sim­il­ar hand con­trol to move the elev­at­or car up or down the shaft, to con­trol the speed and to stop at the appro­pri­ate floor.

Westinghouse Streetcar Controller
1920’s era Westinghouse Streetcar Controller

If the oper­at­or was to doze off or fall uncon­scious, the street­car would simply con­tin­ue on its way until it hit some­thing or derailed, either being a poor option! Elevators would con­tin­ue until they hit the top or bot­tom of the shaft, again a bad idea. Sprague included a con­trol device in his designs that required the oper­at­or to keep his hand on the con­trol­ler handle and to main­tain pres­sure on the con­trol device in order for power to flow to the motor. This same idea was imple­men­ted in manu­al elev­at­or con­trol handles. These ideas were adop­ted by Westinghouse when they developed the street­car motor con­trol­lers that were used in thou­sands of street­cars run­ning between the 1890’s and the 1930’s.

When diesel-​electric and full elec­tric loco­mot­ives were developed in the 1930’s, the concept of the ‘dead­man’ con­trol was adop­ted from street rail­ways. There is a per­sist­ent myth that these con­trols star­ted with steam loco­mot­ives. In fact, an earli­er ver­sion of this art­icle included that myth – now bus­ted!

A 'deadman' pedal in a locomotive.
A ‘dead­man’ ped­al in a diesel-​electric rail­way loco­mot­ive

With the advent of elec­tric trams, trains, and sub­ways, con­cerns about pos­sib­il­it­ies like heart attacks and oth­er infirm­it­ies res­ult­ing in drivers los­ing con­trol of these machines caused these devices to be integ­rated into these new trans­port­a­tion sys­tems. To learn more about these applic­a­tions, see the Wikipedia art­icle Dead Man’s Switch.

It’s worth not­ing that the rail­ways now call these devices ‘Driver Safety Devices’ or DSD. See a mod­ern DSD at the Arrowvale Electronics web site.

Elevators moved from manu­al con­trol to auto­mat­ic con­trol, elim­in­at­ing the need for elev­at­or oper­at­ors and the need for ‘dead­man’ con­trols.

Robots Enter the Picture

Motoman robot pendant enabling device
Motoman pendant with show­ing enabling device (red arrow)

In the 1980’s, indus­tri­al robots began to appear in the work­place. Accidents in these early days drove changes in the design of the con­trol pendants used to ‘teach’ these devices their tasks. Early pendants provided motion con­trol and an emer­gency stop device. Later, the motion con­trols were altered to become ‘hold-​to-​run’ devices that could jog the selec­ted robot axis at a pre-​selected slow-​speed, one axis at a time. In the 90’s the ‘enabling device’ was added to the pendant. These two-​position switches, still called ‘dead-​man switches’, had to be held closed in order for the robot to move under con­trol of the axis hold-​to-​run con­trols. Accidents con­tin­ued to occur. In the mid 90’s the three-​position enabling device, some­times called a ‘live-​man-​switch’, was intro­duced after stud­ies showed that some people would release their grip on the con­trol pendant when struck by the robot, while oth­ers would clench the hand hold­ing the pendant. The new switches are required to be held in the mid pos­i­tion to enable motion. The pic­ture at left shows the back of a mod­ern robot pendant. The black bar in the lower right is the enabling device, loc­ated so that your hand will nat­ur­ally hold the device in the cor­rect pos­i­tion when you hold the pendant in your left hand. Not so good if you are left-​handed!

ABB IRB640 Robot Pendant
ABB IRB640 Robot Pendant

 

 

Euchner ZS Switches

In addi­tion to the pendant enabling devices, addi­tion­al enabling devices are required where more than one work­er is required inside the danger zone of the machine. These devices can be pur­chased sep­ar­ately and added to sys­tems as needed. Depending on the applic­a­tion, you can get these devices with emer­gency stop but­tons and jog but­tons integ­rated into a single unit as shown in the pic­ture of the Euchner ZS switches.

Machinery Standards and Definitions

The enabling device is one of those pro­tect­ive meas­ures that can­not be read­ily clas­si­fied as a safe­guard­ing device because they do not pro­act­ively pre­vent injury. Instead, like an emer­gency stop, they may allow a work­er to avert or lim­it the harm that is already occur­ring. That places the enabling device into the ‘com­ple­ment­ary pro­tect­ive meas­ure’ cat­egory.

Let’s take a minute to look at a couple of import­ant defin­i­tions from the machinery stand­ards. At the moment, the best defin­i­tion for a com­ple­ment­ary pro­tect­ive meas­ure comes from the Canadian stand­ard, CSA Z432-​04. Excerpted from CSA Z432-​04, §6.2.3.5.3 Complementary Protective Measures:

Protective meas­ures that are neither inher­ently safe design meas­ures, nor safe­guard­ing (imple­ment­a­tion of guards and/​or pro­tect­ive devices), nor inform­a­tion for use may have to be imple­men­ted as required by the inten­ded use and the reas­on­ably fore­see­able mis­use of the machine. Such meas­ures shall include, but not be lim­ited to,

a) emer­gency stop;

b) means of res­cue of trapped per­sons; and

c) means of energy isol­a­tion and dis­sip­a­tion.

Let’s also look at the form­al defin­i­tion of an ‘enabling device’ in the same stand­ard:

7.23.3 Enabling devices
7.23.3.1
An enabling device is an addi­tion­al manu­ally oper­ated 2- or 3-​position con­trol device used in con­junc­tion with a start con­trol and which, when con­tinu­ously actu­ated in one pos­i­tion only, allows a machine to func­tion. In any oth­er pos­i­tion, motion is stopped or a start is pre­ven­ted.

7.23.3.2
Enabling devices shall have the fol­low­ing fea­tures:

a) They shall be con­nec­ted to a Category 0 or a Category 1 stop (see NFPA 79).

b) They shall be designed in accord­ance with ergo­nom­ic prin­ciples:

(i) pos­i­tion 1 is the off func­tion of the switch (actu­at­or is not oper­ated);

(ii) pos­i­tion 2 is the enabling func­tion (actu­at­or is oper­ated); and

(iii) pos­i­tion 3 (if used) is the off func­tion of the switch (actu­at­or is not oper­ated past its mid pos­i­tion).

c) Three-​position enabling devices shall be designed to require manu­al oper­a­tion in order to reach pos­i­tion 3.

d) When return­ing from pos­i­tion 3 to pos­i­tion 2, the func­tion shall not be enabled.

e) An enabling device shall auto­mat­ic­ally return to its off func­tion when its actu­at­or is not manu­ally held in the enabling pos­i­tion.

Note: Tests have shown that human reac­tion to an emer­gency may be to release an object or to hold on tight­er, thus com­press­ing an enabling device. The ergo­nom­ic issues of sus­tained activ­a­tion should be con­sidered dur­ing design and install­a­tion of the enabling device.

 

OMRON A4EG Enabling Switches
OMRON A4EG Enabling Switches

Similar defin­i­tions exist in the International, European and US stand­ards, although they may not be quite as form­al­ized.

 

Most enabling devices on their own do noth­ing except PERMIT motion to take place, although the actu­al defin­i­tion of enabling device in CSA Z432-​04 actu­ally per­mits the enabling device to cause motion. Absence of the enabling sig­nal pre­vents or stops motion. These devices are then used in con­junc­tion with hold-​to-​run con­trols on robots and machinery, and with throttle con­trols on trains, street cars, sub­ways and sim­il­ar equip­ment. Note that most stand­ards to not per­mit enabling devices to actu­ally cause motion. This is a unique situ­ation in the Canadian stand­ard.

So what’s the big deal?

Using the terms ‘dead-​man’ or ‘live-​man’ to describe these devices puts the wrong mes­sage out as far as I’m con­cerned. As safety engin­eers and OHS prac­ti­tion­ers, we care about keep­ing work­ers out of danger. This is neither check­ing to see if we have either a ‘dead man’ or a ‘live man’, but rather ensur­ing that the per­son in con­trol of the equip­ment is ‘in con­trol’.  Using a phrase like ‘enabling device’ clearly says what the device does.

In my opin­ion, and  sup­por­ted by the cur­rent International and Canadian Standards, these terms must be aban­doned in favour of ‘enabling device’ and the qual­i­fi­ers ‘2-​position enabling device’ and ‘3-​position enabling device’. These terms are also used in many of the cur­rent machinery safety stand­ards, so using them cor­rectly improves clar­ity in writ­ing and speak­ing. Clarity in com­mu­nic­a­tion in safety is too import­ant for prac­ti­tion­ers to per­mit the ongo­ing use of terms that con­vey the wrong mes­sage and do not pro­mote clar­ity of mean­ing. Since clar­ity is often lack­ing when it comes to safety, any­thing we can do to improve our com­mu­nic­a­tions should be high on our pri­or­ity list!

Ed. note: This post was updated on 17-​Aug-​17. The myth of dead­man con­trols on steam loco­mot­ives was removed and replaced by his­tor­ic­ally veri­fi­able inform­a­tion about the ori­gins of this con­trol.

Author: Doug Nix

+DougNix is Managing Director and Principal Consultant at Compliance InSight Consulting, Inc. (http://www.complianceinsight.ca) in Kitchener, Ontario, and is Lead Author and Managing Editor of the Machinery Safety 101 blog.

Doug's work includes teaching machinery risk assessment techniques privately and through Conestoga College Institute of Technology and Advanced Learning in Kitchener, Ontario, as well as providing technical services and training programs to clients related to risk assessment, industrial machinery safety, safety-related control system integration and reliability, laser safety and regulatory conformity.

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  • ken29

    The prob­lem with his ana­lys­is is that, basic­ally, he is look­ing at it from the oper­at­or view, infer­ring that oper­a­tion or inop­er­a­tion of the machine only impacts the oper­at­or, while the ori­gin­al pur­pose and its applic­a­tion in the con­tem­por­ary dis­aster was to pro­tect oth­ers from the machine when the oper­at­or did not show pos­it­ive con­trol (i.e. “dead”). There really are two dif­fer­ent prob­lems, pos­sibly war­rant two dif­fer­ent names, but “dead man” is cer­tainly appro­pri­ate for the prob­lem that ori­gin­ated the term.

    • Ken29,

      Thanks for your com­ment. I don’t dis­agree entirely – the ori­gin­al pur­pose was to pre­vent a run­away machine from injur­ing oth­ers and des­troy­ing itself. The death or dis­ab­il­ity of the oper­at­or was ancil­lary to pro­tect­ing the machine and oth­ers who might be affected.

      Having said that, the term “dead­man” is no longer used in tech­nic­al stand­ards or in reg­u­la­tions related to safety. These devices have had oth­er names coined for them, depend­ant on the sec­tor. As I men­tion in the art­icle, the rail­ways now call these sys­tems out as “Driver Safety Devices”, while in auto­ma­tion and robot­ics they are termed “enabling devices”. In my opin­ion, both terms are more descript­ive of the func­tion of the device or sys­tem than the old “dead­man”.

      • ken29

        Doug: I cer­tainly agree that “dead­man” by itself isn’t par­tic­u­larly inform­at­ive and “dead­man switch” only slightly bet­ter, but any­thing that ends in “device” is pretty ambigu­ous and prob­ably more suited to stand­ards and legis­la­tion than to pop­u­lar ver­nacu­lar. Not that I have thought of a bet­ter altern­at­ive.

        • ken29,

          I see your point, how­ever the use of the term device allows for new tech­no­lo­gies that may not be acco­mod­ated by “switch”. If you con­sider some of the older devices that have served this pur­pose, valves and switches have both served this pur­pose. Current stand­ards reflect the change to 3-​position devices that incor­por­ate mul­tiple switches and require the user to hold the device in a mid pos­i­tion. This device is designed to ensure that no what reac­tion a user has when injured, the device will remove power from the machinery. Language needs to advance as well as tech­no­logy. It’s my opin­ion that this change, now more than 10 years old, reflects that advance­ment.

          • ken29

            Again, that’s great for tech­ies, stand­ards, and legis­la­tion, but not use­ful or help­ful for every­day com­mu­nic­a­tion with the pub­lic.

          • No argu­ment. This is always the chal­lenge. How can we most effect­ively con­vey import­ant ideas to lay people, while avoid­ing jar­gon? At the same time, how can we effect­ively com­mu­nic­ate com­plex ideas suc­cinctly when deal­ing with oth­er pro­fes­sion­als.

            What I try to do with this blog is to de-​mystify com­plex ideas. My goal is to try to help any­one who is inter­ested in under­stand­ing the key ideas related to safety of machinery. There are a lot of mis­con­cep­tions and con­fu­sion out there – I run into it in my prac­tice all the time. If pro­fes­sion­al people work­ing in this sec­tor are con­fused, I’m sure non-​professionals are too.

            So what’s the altern­at­ive in this case? “Deadman” has been widely used in the past, although it brings with it some ideas that may not be com­pletely cor­rect. “Enabling Device” says what the thing does, but is not as well under­stood. Do you think a third option might exist?

          • ken29

            Doug: This isn’t a com­pletely sat­is­fact­ory, or even sat­is­fy­ing, answer, but it seems to come as close as I’ve been able to devise. How about using “con­trol” instead of device, then adding a word or two (as we usu­ally do) to select the spe­cif­ic con­trol?
            I spent some time Googling “device”, “switch”, and “con­trol” to see how they were rep­res­en­ted in Google (sort of con­tem­por­ary ver­nacu­lar) and con­trol seemed to “win” by some mar­gin. I think we got into this con­ver­sa­tion through the train crash out East and for that applic­a­tion I think a “dead­man con­trol” works fairly well and improves on “dead­man switch”. I saw three-​function “grip” used on an indus­tri­al machine and that matched the pic­ture well, but I think con­trol would work as well.
            Good luck! It’s a worthy cause.

          • Ken,

            I agree, “con­trol” cov­ers it effect­ively and could be used to include any kind of “device” or devices neces­sary for any type of applic­a­tion. Good idea!

            Modern rail­way Driver Safety Devices (DSDs) are more com­plex than the ped­al shown in the pic­ture in the art­icle. That one is pretty old-​school. I’ve heard from a few people that the ped­al shown in the art­icle often ended up with a tool­box hold­ing it down, defeat­ing the pur­pose of the con­trol.

            Now they usu­ally require the driver to act­ively do some­thing with­in a cer­tain peri­od of time, like press and release a but­ton, or move a con­trol, etc. Every applic­a­tion has it’s own unique require­ments based on what the oper­at­or is required to do to use the machine.

  • Roberta Nelson Shea

    Good tim­ing! I was just at the Automate Show, work­ing in the Robot Safety Standard booth. I was demon­strat­ing some new robot cap­ab­il­it­ies, then said “I hold the enabling device in the cen­ter “on” pos­i­tion”. One of the observ­ers chimed in “the dead­man switch”. I care­fully explained the his­tory – good thing it was the same as the art­icle 😉 And then I explained that a dead­man switch was developed to detect a dead man and stop the train so that more people do not die. However the enabling is used to keep people from harm, have con­trol of the sys­tem, and ENABLE the equip­ment to oper­ate with­in para­met­ers (reduced speed, reduced torque, lim­ited time dur­a­tion,…).